The present invention relates to a phase change type of optical recording device which is capable of rewriting information by use of phase change and an optical recording method using the same and, in particular, to a phase change type of optical recording device which radiates a pulse-like laser beam controlled so as to satisfy a given condition when a recording mark is formed to thereby obtain an excellent repetitive recording and erasing characteristic and an optical recording method using the same.
Conventionally, as a recording medium for use in a rewritable optical recording system which radiates a focused light such as a laser beam or energy such as heat on the recording medium to change reversibly the optical properties of the radiated portion of the recording medium to thereby achieve the recording, reproducing or erasing of information, for example, there is known a recording medium which comprises a light transmissive substrate including a pre-groove for tracking or focusing servo, a recording layer disposed on the upper surface of the substrate and formed of a recording material of a phase change type, an inorganic dielectric layer disposed on the opposite side of the recording layer to the substrate, a reflecting layer disposed on the opposite side of the inorganic dielectric layer to the substrate, and another inorganic dielectric layer which can be interposed between the substrate and recording layer as required to protect the recording layer.
Then, as a method for recording or erasing the information on such optical recording medium, there is normally employed a one-beam-over-write recording and erasing method which will be described below:
To write the information into the optical recording medium, a circular laser spot of a high output is driven by a rectangular pulse wave to radiate the recording layer, and the radiated portion of the recording layer is heated to a temperature equal to or higher than the melting temperature (Tm) of the recording material and is then quenched, thereby changing the radiated portion from its crystal state (crystal phase) to its noncrystal state (amorphous phase). On the other hand, to erase the written information, a circular laser spot of a low output is driven by a rectangular pulse wave to radiate the recording layer, and the radiated portion of the recording layer is heated within a range of temperature equal to or higher than the crystallizing temperature (Tx) of the recording material in the radiated portion of the recording layer and equal to or lower than the melting point thereof and is then cooled down gradually, thereby changing the radiated portion of the recording layer from its noncrystal state (amorphous phase) to its crystal state (crystal phase).
As the recording material that can be used to form the recording layer of an optical recording medium utilizing the above-mentioned phase change between the crystal phase and amorphous phase, in particular, there is known a thin film material such as material of Ge--Sb--Te system, material of In--Ge--Te system or the like. For example, it is reported about the material of Ge--Sb--Te system that a time necessary for erasure is 100 ns or less and thus a high speed erasure is possible, and its crystallizing temperature is 150.degree. C. or higher and thus its stability in its amorphous phase is high (see "CPM 87 to 88 (1987)" published by Electronic Communication Society).
However, in the conventional optical recording system of a phase change type which employs the above-mentioned one-beam-over-write recording and erasing method, there is a great problem in its repetitive recording and erasing characteristic. That is, as the number of times of recording, reproducing and erasing is increased, noise is increased, which deteriorates the recording and reproducing characteristic of the system.
As a main factor which has an effect on the repetitive characteristic, there are pointed out the movement of the recording material caused by a heat history and the thermal deterioration of the protective layer and reflecting layer. Here, the movement of the recording material is a phenomenon that the recording material in the portion of the recording layer heated and melted by means of radiation of a laser beam is caused to move gradually along a track in the advancing direction of an optical disc (an optical recording medium) or in the reversed direction thereof when recording and erasing are repeated. Describing the movement phenomenon concretely, as shogun in FIGS. 23 and 24, when a laser spot light 31 driven by a rectangular pulse wave 30 is used to radiate a recording layer 22 (an arrow A in FIGS. 23 and 24 shows the moving direction of the laser spot light), it is believed that there occurs a movement (FIG. 23) caused by the asymmetrical deformation of the recording material and a movement (FIG. 24) caused by the asymmetrical solidification of the recording material (the states of such movements are shown along the radiation processes of the laser spot light sequentially, that is, in FIGS. 23 and 24, there are shown sequentially the respective states thereof in the black dot positions respectively shogun in the rectangular pulse wave 30). It is believed that the movement (FIG. 23) of the recording material caused by the asymmetrical deformation thereof is a phenomenon which occurs as a result of asymmetric thermal expansion induced in the material due to the uneven distribution of heat generated when it is heated. In FIG. 23, reference character p designates the highest heat portion, q a high heat portion and r a melted area. Also, it is believed that the movement (FIG. 24) caused by the asymmetrical solidification is a phenomenon which occurs as a result of the fact that the material of the melted area is pushed out by the earlier solidifying portion due to a time lag between the starting times of the material solidification after the material is heated and melted. In FIG. 24, reference character s designates a solidification start portion.
When the recording layer is caused to vary in the thickness thereof with the above-mentioned movements of the recording material, then in the portion of the recording layer with the thickness thereof reduced, there is increased the area of the interface thereof with respect to the volume of the recording material, which makes it easy for heat to escape. This results in the deteriorated sensitivity of the thickness reduced portion and the mechanical strength of the thickness reduced portion is decreased, which facilitates the generation of defects in such portion. On the contrary, in the thickness increased portion, the energy absorbed therein is difficult to escape and thus quenching becomes impossible, with the result that a recording mark is reduced in size to thereby lower C/N. Further, when the thicknesses of the recording layer are reduced or increased to be out of the design thicknesses of the recording layer, then the reflectance thereof is caused to vary, which results in the deteriorated signals. As a result of this, there are generated the above-mentioned increase of the noise and the deterioration of the recording and reproducing characteristic. In the worst case, no recording material is present in the neighborhood of a data recording starting point and thus recording itself is impossible.
In order to solve these problems, models for thermal analysis and material movement have been proposed. For example, with respect to the thermal analysis, Ohta et al. report their examination in "CPM 89-84" published by Electronic Communication Society. Also, in "SPIE Vol. 1078 Optical Data Stage Topical Meeting (1989)", there is proposed a model for the movement of the recording material due to the deformation of the protective film in the recording and melting time. However, no practical means has been found yet which prevents the above-mentioned movement of the recording material.
As concrete means to control the movement of the recording material to thereby improve the repetitive recording and erasing characteristic of the optical recording device, there has been proposed an optical recording member in which each of heat resistant protective films respectively to be disposed on the top and bottom surfaces of a recording layer formed of a recording material of a phase change type is formed of a material of which the quantities of expansion are symmetrical to each other in the front and rear portions of a mark to be formed in the heat resistant protective film by radiating a laser beam (Japanese Patent Unexamined Publication No. Hei. 2-195,538). However, according to this optical recording member, the repetitive recording and erasing characteristic cannot be improved sufficiently and the material of the heat resistant protective film is limited. Also, in this recording member, in order to improve the symmetry of the thermal expansion quantities in the front and rear portions of the mark in the heat resistant protective layer, as a laser beam used to radiate the recording layer to thereby form the recording mark, there is employed means using a pulse wave which has a power level at the starting time of the pulse radiation higher (for example, 50% or so) than a power level at the completion time of the pulse radiation. However, this laser beam radiation means is found that it is not able to improve the repetitive recording and erasing characteristic sufficiently.
Besides the above proposals, in an Optical Disc Meeting in 1991, Suzuki et al. proposed that a medium structure should be employed in which the highest temperature is present at the center of a melting area when a recording material is melted. In fact, however, it is difficult to provide sufficient improvement effects even if this means is employed.
Also, as an optical recording system of the above-mentioned phase change type, there is known a multi-pulse recording system which is pit edge recording system and which radiates a plurality of rectangular pulses each having a short pulse width so as to record a recording mark. The multi-pulse pit edge recording system is a system which detects the positions of both ends of a recording mark to thereby reproduce recorded information. When compared with the conventional pit position recording system which detects the central position of a recording pit, the multi-pulse pit edge recording system has a recording density 1.5 to 2 times and, for this reason, the pit edge recording system is now under serious study. Among the pit edge recording systems, especially, a multi-pulse recording system becomes the object of the study. While the conventional pit edge recording system radiates a rectangular pulse of a length corresponding to the length of a mark to be formed and then forms the mark, the multi-pulse recording system radiates a plurality of pulses each having a short pulse width to thereby form a recording mark (Japanese Patent Unexamined Publication No. Sho. 63-300436, Optical Memory Symposium '90 pp. 77-78). According to the multi-pulse recording system, a recording medium is not heated higher than necessary in forming a recording mark and thus a thermal load can be reduced. In such multi-pulse recording system as well, with respect to its repetitive recording and erasing characteristic, there exists a similar problem which is caused by the movement of the recording material or the like.